Cold-cathode field emission electron sources are based on room-temperature, field-enhanced tunnelling at the apex of a sharp-tipped structure (Fowler and Nordheim 1928). The development of the first practical devices is due to Spindt (Spindt 1968; Spindt et al. 1976; U.S. Pat. No. 3,665,241). These devices were based on cylindrically symmetric sharp tips formed by etching in a material with low work function. Since then, there has been considerable further development of silicon-based Spindt emitters for applications in vacuum microelectronics (Cade et al. 1990; Jones et al. 1992), vacuum instruments (Itoh 1997), electron beam lithography (Hofmann et al. 1995) and thin-film displays (Gorfinkel et al. 1997).
FIG. 1 shows the most common geometry for a field-emission triode. Here a sharp tip etched in a conducting substrate acts as the cathode or electron emitter. A planar conductive layer spaced from the substrate by a thin, high quality layer of insulator material acts as the gate or control electrode. A separate conductive layer acts as the anode or electron collector. Electron emission takes place vertically, when a high field is applied between the gate and the cathode under vacuum. The majority of the extracted electrons normally reach the anode, so that the anode current IA usually exceeds the gate current IG by a large factor.
The tips are conventionally fabricated by isotropic plasma etching of single-crystal silicon using gases such as SF6, although actual emission may take place from other deposited materials such as diamond-like carbon (Lee et al. 1997; Huq 1998). To obtain a high field, extremely small tip radii and small cathode-gate electrode separations are required. Methods of forming suitable tip radii based on oxidation machining have been developed (Marcus et al. 1990; Liu et al. 1991; Huq et al. 1995). Methods of fabricating closely spaced gates and focusing electrodes have also been developed (U.S. Pat. Nos. 5,266,530; 5,228,877, Itoh et al. 1995). Since the required electrode separation is normally very small, the definition of the electrodes often involves a process that avoids lithography and that has inherent self-alignment.
Less attention has been paid to knife-edge or wedge-shaped emitters, because of the reduction in electric field strength arising from the elimination of one radius of curvature from the emitter tip (Chin et al. 1990). However, knife-edge emitters offer potentially high emission current due to their large emission area. Furthermore, there is considerable flexibility in the choice of cathode material when the emitter is constructed from a deposited thin film, and low work function materials other than silicon may be used.
Knife-edge emitters have been constructed with both horizontal (in-plane) and vertical (out-of-plane) cathodes. In some horizontal structures, an entirely in-plane arrangement of cathode and gate electrodes has been adopted (Hoole et al. 1993; Gotoh et al. 1995). In these cases, the required small electrode separation was obtained by electron-beam lithography (in the first case) and focussed-ion-beam etching (in the second). In another horizontal structure, the gate and cathode electrodes were arranged in a planar stack, as shown in FIG. 2 (Johnson et al. 1997). In this case, the required small electrode separation was obtained using thin deposited insulator layers.
A number of vertical cathode structures have been constructed in silicon. For example, FIG. 3 shows an emitter based on a wedge-shaped silicon cathode (Jones et al. 1992). This structure is conceptually similar to the cylindrical emitter previously shown in FIG. 1. Techniques have been developed to sharpen the tip of the silicon wedge, for example, by oxidation machining (Liu et al. 1991) or by preferential erosion of a surface mask layer (Rakshandehroo et al. 1996).
Similarly, a number of vertical or partially vertical cathode structures have been constructed from metal layers deposited on silicon substrates. The advantage of using a metal layer is that a small tip radius can be achieved without special processing, since the maximum tip radius cannot exceed half the thickness of the metal layer. For example, FIG. 4 shows a petal-shaped field emitter, in which the metal layer is deposited through a self-aligned circular mask onto the sloping walls of a pyramid-shaped pit formed by anisotropic etching of silicon (Gamo et al 1995). A number of related devices known as volcano emitters have been described (Wang et al. 1996; Lee et al. 1997).
FIG. 5 shows a volcano emitter based on a vertical wall formed in a thin layer of silicon carbide (Busta 1997; U.S. Pat. No. 6,008,064). The exposed vertical tip is obtained by conformally depositing thin layers of silicon dioxide, silicon carbide and a metal on an etched silicon mesa (step 1), and then using chemical mechanical polishing (CMP) to remove the layers from the upper surface of the mesa (step 2). The silicon dioxide is then recessed by wet chemical etching to improve the electrical isolation (step 3). In this case, the silicon substrate acts as the gate, the silicon dioxide as the insulator and the silicon carbide as the cathode.
The principle of material deposition over an etched substrate has been used as a method of fabricating vertical-wall emitters by many others, particularly Hsu and Gray (Hsu et al.1992; Hsu et al.1996; U.S. Pat. Nos. 4,964,946; 5,214,347; 5,266,155; 5,584,740; 6,084,245; 6,168,491; 6,246,069).
For example, FIG. 6 shows the formation of a vertical metal wall by depositing a single layer of metal over a cylindrical etched mesa (steps 1 and 2). The metal film is then removed from the upper surface of the mesa by ion bombardment (step 3). In this case, the ion bombardment was continued to remove the entire mesa structure to obtain a free-standing annular vertical metal wall (step 4). Multi-layer deposition of metals and insulators may again be used to obtain more complex layered vertical electrode structures. Clearly, the main difference from the work of Busta is the use of ion-beam erosion instead of chemical mechanical polishing, which cannot easily form such free-standing structures.
Fleming has devised an entirely different field-emission device containing both vertical and horizontal metal electrodes (Fleming et al. 1996; U.S. Pat. No. 5,457,355). FIG. 7 shows one process for forming such a structure. Successive layers of silicon dioxide, silicon nitride and titanium nitride are first deposited on a silicon substrate, and a trench is etched through all these layers to the substrate (step 1). Further layers of polysilicon, titanium nitride and silicon dioxide are then deposited over the trench (step 2). The silicon dioxide is then etched in a reactive plasma, whose action is stopped at the TiN layer (step 3).
The exposed, upper layer of TiN is then etched in a wet acid etch, so that the horizontal upper TiN layer is removed and the vertical TiN layer is slightly recessed (step 4). The exposed polysilicon layer is then removed by extended etching in an isotropic plasma-etch process, for example based on SF6. Finally, the exposed silicon dioxide layer is recessed by wet chemical etching in hydrofluoric acid to improve the electrical isolation (step 5).
In this structure, the vertical TiN layers act as cathodes, and the upper horizontal layer of TiN provides a set of gate electrodes. However, these two electrode types are formed from films deposited by successive and different deposition steps. The only lithographic step used is the process defining the initial etched trench opening. The subsequent electrode alignment and a small electrode separation are achieved through the use of inherently self-aligned processing based on multi-layer deposition over the etched structure followed by selective etching.